In the presence of hydrogen peroxide, chloride ions and chloroperoxidase (CPO), three different substrates, malonic acid, acetoacetic acid and 3-oxooctanoic acid have been shown to produce chloroform. In addition, the formation of methylene chloride and carbon tetrachloride have been demonstrated. A preliminary study of the effect of pH was conducted and the amount of chloroform production apparently is a direct function of enzyme activity. The maximum amount of chloroform generated is obtained near the maximum pH for CPO activity. If chloroform was generated by the "haloform" reaction only, an increase of the halomethane concentration would be expected with increasing pH. Clearly, this is not the case and leads to further speculation concerning the mechanism leading to formation of chloroform. Several phenols were also investigated as possible precursors of chlorophenols. Phenol itself yielded a mixture of ortho- and para-chloro isomers when incubated with CPO, H2O and C1-. The methyl substituted phenols (o-, m- and p- cresols) yielded the corresponding chloro cresols. These reaction products were identical to those expected from treatment of the cresols with hypochlorous acid. The results indeed demonstrate that halogenated substances found in the biosphere can be formed enzymatically. Thus, some of the substances classified as environmental pollutants may also arise by natural metabolism.